Electroslag Fusion Process for Manufacturing a Blade Slab having a Large Curved Surface

ABSTRACT

The invention provides an electroslag fusion process for manufacturing a blade slab having a large curved surface, and it is more particularly effectively in making a blade slab which has a big width-to-thickness ratio, a large difference between the thin and the thick edges and a large curved surface. Firstly, dividing the blade slab into two or three regions according to the external shape and the sectional size of the blade slab, wherein the region which has difficulty in unilateral or bilateral mold-filling is pre-fabricated by the electroslag casting technology to produce a pre-fabricated curved slab, and then it is placed in advance in a side of an internal cavity of a mold, and then fusing the molten metal melted from a consumable electrode and one or two electroslag pre-fabricated slabs which are placed in advance in the mold by utilizing the electroslag fusion process, so as to produce the blade slab having a large curved surface. The large curved blade slab prepared by the process of the present invention has good internal and surface qualities, which can improve material utilization rate, shorten the processing period and improve quality, and in particular, has high anti-fatigue performance, high crack resistance and extensibility performances. The process of the present invention is more suitable for producing large or very large curved blade slab castings having a width-to-thickness ratio &gt;10 and a single weight over 10 tons.

BACKGROUND OF THE PRESENT INVENTION Field of Invention

The present invention relates to a process for manufacturing a castinghaving a big width-to-thickness ration and a large curved surface, andmore particularly to an electroslag fusion process for manufacturing ablade slab having a large curved surface.

Description of Related Arts

In the field of equipment manufacture, a variety of irregular wide andthick plate castings and/or forgings are often used, which arecharacterized by a big width-to-thickness ration and an irregularcross-section shape. As one of the important basic components ofhigh-level equipments, the quality requirements of this special-shapedplate casting are extremely strict. For example: the quality problem ofa turbine blade casting almost decides the overall running life of theturbine.

At present, the methods for manufacturing such castings and forgings aremainly as follows: conventional sand casting process, forging processand electroslag fusion process, at home and abroad.

(1) Due to the limitations of the process itself, the conventional sandcasting products are often difficult to meet the requirements of use,resulting from its poor internal quality; (2) the wide-thick platecastings and forgings produced by the forging process have good internalqualities, but the forging process has a very low metal utilizationlevel and a sharp increase in production cost and time, and against theconcept of green manufacturing; (3) the wide-thick plate castings andforgings meeting the requirements can be directly produced by anelectroslag casting process, according to its product profile, whichhave excellent solidification quality and mechanical properties, andreach the quality standard of the same material wrought products. Atpresent, the inventor has produced a compact of overall blade by theelectroslag casting method and obtained an invention patent (anelectroslag fusion method for manufacturing a compact of a turbineblade, Publication Number: CN 104174834A). However, the process is onlyeffective in manufacturing plate slab castings having a relatively smallwidth-to-thickness ration, and when it is used for manufacturing largeplate slab castings (width to thickness ratio >10) having complicatedcross-sectional shapes, the molten metal produced by the electroslagcasting cannot fully overflow to the farthest thin edge area, limited byits consumable electrode fabrication and mold cooling conditions, whichresults in poor quality of the thin edge of the casting and limits itsfurther application.

In addition to the above three methods for manufacturing wide and thickplate castings and forgings, Japanese Patent (JP 1999019791A) andChinese Patent (Publication No.: CN 102029378A) respectively propose anew method of welding two original blanks together by an electroslagprocess. However, these two methods are only suitable for the welding ofsteel plates or ingots having an equal thickness, and it is verydifficult to apply to the casting having a large width-to-thicknessratio and an irregular cross-sectional shape.

SUMMARY OF THE PRESENT INVENTION

The invention provides an electroslag fusion process for manufacturing ablade slab having a large curved surface, and it is more particularlyeffectively in making a blade slab which has a big width-to-thicknessratio, a large difference between the thin and the thick edges and alarge curved surface, wherein the method mainly fuses the molten metalmelted by a consumable electrode and one or two electroslagpre-fabricated slabs which are placed in the mold in advance byutilizing the electroslag fusion process, so as to produce a plate slabcasting having a large curved surface.

The technical scheme of the present invention is as follows:

An electroslag fusion process for manufacturing a blade slab having alarge curved surface, which is characterized in that, comprising thefollowing steps: firstly, dividing the blade slab into two or threeregions according to the external shape and the sectional size of theblade slab (see FIGS. 1-2), wherein the region which has great sectionalthickness changes and difficulty in unilateral or bilateral mold-fillingis pre-fabricated by the electroslag casting technology to produce apre-fabricated curved slab, and then is placed in advance in a side ofan internal cavity of a mold; and then fusing the molten metal meltedfrom a consumable electrode and one or two electroslag pre-fabricatedslabs which are placed in advance in the mold by utilizing theelectroslag fusion process, so as to produce the blade slab having alarge curved surface.

The large curved blade slab castings prepared by the process of thepresent invention have good internal and surface qualities, which canimprove material utilization rate, shorten the processing period andimprove quality, and in particular, has high anti-fatigue performance,high crack resistance and extensibility performances. The process of thepresent invention is more suitable for producing large or very largecurved blade slab castings having a width-to-thickness ratio >10 and asingle weight over 10 tons.

Specific steps of the electroslag fusion process are as follows:

(1) controlling the main slag components and the content of theelectroslag fusion system to be as follows: mass percentage of CaF₂:50-62%, Al₂O₃: 25-35%, CaO: 3-8% and a trace component: no more than 5%of the total mass of the electroslag fusion system, so as to define amulti-element electroslag fusion system, wherein the trace component isone or more of MgO, SiO₂ and TiO₂, and the thickness of the slag layeris 12 to 25% of the equivalent diameter of the curved blade slab mold;

(2) the process parameters of the electroslag fusion process are asfollows: Voltage: 70˜120V, current density: 20000˜60000 A/m²;

(3) the feeding process: employing the intermittent feeding, in feedingperiod, firstly lowing the normal current to the minimum feeding currentwithin 2 to 5 minutes at a constant speed, maintaining for 1 to 2minutes, and then rising the current from the minimum feeding current to70 to 80% of the normal casting current within 3 minutes at a constantspeed, repeating the cycle 4 to 5 times, and each time the maximumcurrent is 70 to 80% of the previous maximum current, and at the lasttime, reducing the current to zero.

The electroslag fusion process of the present invention, which is usedfor manufacturing a blade slab having a large curved surface, ischaracterized in that the electroslag casting process can fuse theunilateral pre-fabricated slabs and the bilateral pre-fabricated slabs.

The electroslag fusion process of the present invention, which is usedfor manufacturing a blade slab having a large curved surface, ischaracterized in that: since the pre-fabricated curved surface slab isrelatively complicated, the consumable electrode for electroslag castingis a sand casting electrode, and the consumable electrode of theelectroslag fusion process is a steel welding electrode. According tothe size of the internal cavity of the blade slab mold, the fillingratio of the consumable electrode is 0.15 to 0.4.

The electroslag fusion process of the present invention, which is usedfor manufacturing a blade slab having a large curved surface, ischaracterized in that: the alloy materials of the blade slab having alarge curved surface comprises: low carbon martensitic stainless steel06Cr₁₃Ni₄Mo, 06Cr₁₃Ni₅Mo or 06Cr₁₆Ni₅Mo; ultra-low carbon martensiticstainless steel 04Cr₁₃Ni₄Mo or 04Cr₁₃Ni₅Mo.

Compared with the prior art, the present invention has the followingadvantages:

(1) The electroslag casting is a kind of special casting technologywhich combines refining, solidification and forming. It can furtherremove the inclusions in the consumable electrode and adjust itsdistribution, reducing P, S and other harmful elements and N, H, O andother gas content in the curved blade slab and improving the materialpurity thereof by adjusting the compositions of the electroslag andoptimizing the process.

(2) the electroslag fusion blade slab can achieve the same mechanicalproperties and testing indicators of the same material forging and has ahomogeneity of the chemical composition, dense tissue, no loose,shrinkage and other defects, dispersed inclusions, has high anti-fatigueperformance, high crack resistance and extensibility performances.

(3) the curved blade slab is solidified in the mold, with high coolingspeed, high crystallizer size and crystallizer accuracy, smalldeformation, so the external surface of the casting is smooth and has asmall allowance, so as to achieve near-net shape forming.

(4) The electroslag fusion process is employed to effectively solve theproblem of filling quality of large-scale blade slabs, and the preparedlarge-surface blade slab castings have good internal and surfacequalities. And the process is more suitable for producing large or verylarge curved blade slab castings having width-to-thickness ratio >10 andweight over 10 tons.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural schematic diagram of a unilateral fusionpre-fabricated slab.

FIG. 2 is a structural schematic diagram of a bilateral fusionpre-fabricated slab.

FIG. 3 is a flow chart of the electroslag fusion process formanufacturing a blade slab having a large curved surface.

Among the above FIGS. 1-3, the figure numbers respectively represent: 1:transformers, 2: combined water-cooled mold, 3: electrode, 4: bottomtank, 5: pre-fabricated slab I, 6: pre-fabricated slab II, 7:electroslag fusion region, 8: metal pool, 9: liquid slag.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Example 1

Referring to FIGS. 2 and 3, the electroslag fusion process of thepresent invention, which is used for manufacturing a blade slab having alarge curved surface, selects the martensitic stainless steel06Cr₁₃Ni₄Mo as the original material of the blade slab, wherein themaximum width of the blade is about 4300 mm, the maximum height of theblade is 3400 mm, the maximum thickness of the blade is 310 mm, and theminimum thickness of the blade is 65 mm.

Firstly, a pre-fabricated slab I 5 and a pre-fabricated slab II 6 arerespectively placed in a region I and a region III inside the mold, thetransformer 1 is connected to the bottom tank 4 through a wire, theregion II and the electrode 3 define a loop, the electrode 3 is meltedby resistance heat resistance from a liquid slag 9, the liquid metalpasses through the slag pool and defines a metal pool 8, and thepre-fabricated slab I 5 and the pre-fabricated slab II 6 are fusedtogether by a strong water-cooled mold (the copper plate provided at theinner wall of the mold has a certain angle spiral groove for increasingthe cooling strength). Finally, the region I, the region II and theregion III form the curved blade slab meeting the experiment required.

According to the size of the region II of the internal cavity of themold, the steel plate electrode having a corresponding shape isprepared, wherein the selected filling ratio is 0.3; {circle around (1)}The proportion of the components of the slag system is: CaF₂: 60%,Al₂O₃: 32%, CaO: 6%, MgO: 2%, so as to define a quaternary slag system,wherein the thickness of the slag layer is 20% equivalent diameter ofthe curved blade slab mold; {circle around (3)} power supply parameters:power voltage is determined to be 115V, the current density isdetermined to be 22000 A/m², according to the size of the casting, thegeometric parameters of the electrode and the mold and the castingprocess; {circle around (4)} during the feeding period, reducing thenormal current to a minimum feeding current within four minutes at aconstant speed, maintaining for 1 minute, and then rising the currentfrom the minimum feeding current to 70% of the normal casting currentwithin 2 minutes at a constant speed, repeating the cycle 4 times, andeach time the maximum current is 70% of the previous maximum current,and at the last time, reducing the current to zero.

The large-scale curved blade slab prepared by the process of the processinvention has a dimension conforming to the drawing requirements andgood internal and external qualities. The mechanical properties of thecompact treated by the conventional heat treatment are RP0.2/650 Mpa,Rm/845 Mpa, A/26%, Z/62%, KV2/101J.

Example 2

This example illustrates manufacturing of a X-shaped curved blade slabof a power plant, the materials is martensitic stainless steel06Cr16Ni5Mo. The maximum width of the prepared blade slab is about 2600mm, the maximum height of the blade is 2100 mm, the maximum thickness ofthe blade is 220 mm, and the minimum thickness of the blade is 30 mm.

Firstly, a pre-fabricated slab is placed at an inner side of the mold,the electrode is melted by resistance heat resistance from a liquidslag, the liquid metal passes through the slag pool and defines a metalpool, and then the liquid metal and the pre-fabricated slab placed inthe mold in advance are fused together by a water-cooled mold, so as toform the curved blade slab meeting the experiment required.

According to the remaining size of the mold, the steel plate electrodehaving a corresponding shape is prepared, wherein the selected fillingratio is 0.26; {circle around (2)} The proportion of the components ofthe slag system is: CaF₂: 60%, Al₂O₃: 35%, CaO: 5%, so as to define aternary slag system, wherein the thickness of the slag layer is 21%equivalent diameter of the curved blade slab mold; {circle around (3)}power supply parameters: power voltage is determined to be 85V, thecurrent density is determined to be 35000 A/m², according to the size ofthe casting, the geometric parameters of the electrode and the mold andthe casting process; {circle around (4)} during the feeding period,reducing the normal current to a minimum feeding current within threeminutes at a constant speed, maintaining for two minute, and then risingthe current from the minimum feeding current to 70% of the normalcasting current within 2 minutes at a constant speed, repeating thecycle 4 times, and each time the maximum current is 70% of the previousmaximum current, and at the last time, reducing the current to zero.

The prepared large-scale curved blade slab is tested by profile line andits size meets the requirements of the drawing, and has no ditching onthe surface. And the mechanical properties of the compact treated by theconventional heat treatment are RP0.2/634 Mpa, Rm/810 Mpa, A/25%, Z/55%,KV2/105J, which can meet the requirements of use.

The above embodiments are only used for illustrating the technicalconcept and features of the present invention to enable those skilled inthe art to understand and implement the present invention, and they arenot intended to limit the protection scope of the present invention. Allequivalent changes or modifications made according to the spirit of thepresent invention shall fall within the protection scope of the presentinvention.

What is claimed is:
 1. An electroslag fusion process for manufacturing ablade slab having a large curved surface, comprising the followingsteps: dividing the blade slab into two or three regions according tothe external shape and the sectional size of the blade slab, wherein theregion which has great sectional thickness changes and difficulty inunilateral or bilateral mold-filling is pre-fabricated in advance by anelectroslag casting technology to produce a pre-fabricated curved slab;placing the pre-fabricated curved slab in advance at a side of aninternal cavity of a mold; and fusing a molten metal melted from aconsumable electrode and one or two electroslag pre-fabricated slabswhich are placed in advance inside the mold together, so as to producethe blade slab having a large curved surface.
 2. The electroslag fusionprocess, as recited in claim 1, wherein the specific steps for theelectroslag fusion process are as follows: (1) controlling the maincomponents and the content of the electroslag fusion system to be asfollows: mass percentage of CaF₂: 50-62%, Al₂O₃: 25-35%, CaO: 3-8% and atrace component: no more than 5% of the total mass of the electroslagfusion system, so as to define a multi-element electroslag fusionsystem, wherein the trace component is one or more of MgO, SiO₂ andTiO₂, and the thickness of the slag layer is 12 to 25% of the equivalentdiameter of the curved blade slab mold; (2) the process parameters ofthe electroslag fusion process are as follows: voltage: 70˜120V, currentdensity: 20000˜60000 A/m²; (3) the feeding process: employing theintermittent feeding, in feeding period, firstly lowing the normalcurrent to a minimum feeding current within 2 to 5 minutes at a constantspeed, maintaining for 1 to 2 minutes; and then rising the current fromthe minimum feeding current to 70 to 80% of the normal casting currentwithin 3 minutes at a constant speed, repeating the cycle 4 to 5 times,and each time the maximum current is 70 to 80% of the previous maximumcurrent, and at the last time, reducing the current to zero.
 3. Theelectroslag fusion process, as recited in claim 1, wherein theelectroslag casting process is capable of fusing either the unilateralpre-fabricated slabs, or fusing the bilateral pre-fabricated slabs. 4.The electroslag fusion process, as recited in claim 1, wherein theconsumable electrode for electroslag casting is a sand castingelectrode, the consumable electrode of the electroslag fusion process isa steel welding electrode, and the filling ratio of the consumableelectrode is 0.15 to 0.4.
 5. The electroslag fusion process, as recitedin claim 1, wherein the alloy materials of the blade slab having a largecurved surface comprises: low carbon martensitic stainless steel06Cr₁₃Ni₄Mo, 06Cr₁₃Ni₅Mo or 06Cr₁₆Ni₅Mo, and ultra-low carbonmartensitic stainless steel 04Cr₁₃Ni₄Mo or 04Cr₁₃Ni₅Mo.